Skin cancer is the most commonly diagnosed cancer in the United States. Melanoma accounts for less than 5% of skin cancer cases but causes greater than 75% deaths of skin cancer. It was predicted that 68,720 new cases would be diagnosed and 8,650 deaths would occur in 2009 (1). Early diagnosis and prompt surgical removal are a patient's best opportunity for a cure since no curative treatment exists for metastatic melanoma. Despite the clinical use of 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG) for positron emission tomography (PET) diagnosis and staging of melanoma, [18F]FDG is not melanoma-specific imaging agent and is also not effective in imaging small melanoma metastases (<5 mm) and melanomas that have primary energy sources other than glucose (2-4). Alternatively, melanocortin-1 (MC1) receptor is a distinct molecular target due to its over-expression on both human and mouse melanoma cells (5-9). Radiolabeled α-melanocyte stimulating hormone (α-MSH) peptides can bind the MC1 receptors with nanomolar binding affinities (10-18) and represent a class of promising melanoma-specific radiopharmaceuticals for melanoma imaging and therapy.
Recently, the inventors have developed a novel class of 111In-labeled lactam bridge-cyclized DOTA-conjugated α-MSH peptides for melanoma detection (19, 20). Lactam bridge-cyclization was employed to improve the stabilities of the α-MSH peptides against the proteolytic degradations in vivo and enhance the binding affinities of the α-MSH peptides through stabilizing their secondary structures such as beta turns (21-24). The radiometal chelator DOTA was attached to the N-terminus of the lactam bridge-cyclized α-MSH peptide (12-amino acids in the peptide ring) for 111In radiolabeling. For instance, 111In-DOTA-GlyGlu-CycMSH (DOTA-Gly-Glu-c[Lys-Nle-Glu-His-DPhe-Arg-Trp-Gly-Arg-Pro-Val-Asp]) exhibited high MCI receptor-mediated tumor uptake (10.40±1.40% ID/g at 2 h post-injection) in flank B16/F1 melanoma-bearing C57 mice (19). Both flank primary and pulmonary metastatic melanoma lesions were clearly visualized by small animal SPECT/CT using 111In-DOTA-GlyGlu-CycMSH as an imaging probe (19, 20), highlighting its potential as an effective imaging probe for melanoma detection.
One advantage of the lactam bridge-cyclized α-MSH peptide is that the peptide ring size can be finely modified by either adding or deleting amino acids without sacrificing the binding affinity of the peptide (19, 20). The studies on the α-MSH peptide agonists for the MC1 receptor revealed that the lactam bridge-cyclized α-MSH peptide with a 6-amino acid peptide ring {Ac-Nle-c[Asp-His-DPhe-Arg-Trp-Lys(CONH2)], MT-II}displayed not only higher MC1 receptor binding affinity, but also slower MC1 receptor dissociation rate than the native α-MSH peptide {Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2}(SEQ ID NO: 1) (25, 26). Slow MC1 receptor dissociation rate might contribute to the prolonged biological activity of MT-II in vitro and in vivo (25). In this study, we conjugated the radiometal chelator DOTA to the N-terminus of the MT-II peptide to generate a novel DOTA-conjugated lactam bridge-cyclized α-MSH peptide with a 6-amino acid peptide ring (DOTA-Nle-CycMSHhex) to examine the effect of peptide ring size on its melanoma targeting and pharmacokinetic properties. The MCI receptor binding affinity of DOTA-Nle-CycMSHhex was determined in B16/F1 melanoma cells. DOTA-Nle-CycMSHhex was radiolabeled with 111In which is a commercial available diagnostic radionuclide with a half-life of 2.8 days. The melanoma targeting and pharmacokinetic properties and SPECT/CT imaging of 111In-labeled DOTA-Nle-CycMSHhex were determined in B16/F1 melanoma-bearing C57 mice.